1. Field of the Invention
The present invention relates to a metal mold suitable for the press molding of a thermoplastic resin and a process for press molding using the mold.
2. Description of Related Art
A great deal of plastic moldings are widely used in any field such an automobile and a domestic electrical equipment due to their economy, ease of shaping and light weight;
A press molding process or an injection molding process is known as a process for the production of such moldings. The press molding process can produce moldings having less molding strain since they can be molded under a lower pressure than that of the injection molding process so that moldings with less warpage and deformation can be advantageously produced at a low cost using a molding machine having a small clamping force. In addition, the press molding process is also advantageous in that when various materials are laminated on surfaces of the moldings, the skin materials are unlikely to be damaged since the molding pressure is smaller.
For example, Japanese Patent Kokai Publication No. 22917/1986 (corresponding to U.S. patent application Ser. No. 7/793,329) discloses such a press molding process of a thermoplastic resin in which the thermoplastic resin in molten condition is supplied through a molten resin passage which is provided in a metal mold body and the resin is pressed. However, the mold used in the process has a nozzle, as described in the Publication, such that a tip portion of the nozzle through which the resin goes into an inside of the mold is flush with a cavity side surface of the mold (see FIG. 1). Thus, when molding is repeated using such a mold, a temperature of a portion of the mold near the nozzle is gradually increased by the supplied molten resin. Especially, when the molding is repeated in a short time (namely, a molding cycle is short), such a portion of the mold is insufficiently cooled so that a molded article has a bulge in its portion which has been adjacent to such an insufficiently cooled portion and/or the molded article has a gloss, on such a portion, which is different from that of other portion of the molded article. It is, therefore, difficult to produce an article having a good appearance.
In addition, when a multi-layer molded article comprising a skin material laminated thereon is formed, the article has a bulge on a portion thereof which was adjacent to or near the nozzle, so that the molded article having a good appearance cannot be produced by a series of molding.
When a temperature of the nozzle portion of the mold is set to a relatively lower temperature so as to prevent the temperature increase of the mold surface portion near the nozzle, not only but difficult for the molten resin to enter the cavity of the mold, but also that a temperature of the molten resin is reduced so that it is likely to produce a defective article such as a molded article having a ring-shaped mark on a portion thereof which corresponds to an inlet through which the molten resin is supplied into the cavity of the mold.